Capacitance and Charge

I love this essay on capacitance and charge.
Does the word charge describe accuratly the process of a capacitor.
Is Energy a better word?
I think will all my study of antigravity, magnatism, and capacitance...that he is on too something.

Think about this:

When "charging" a capacitor, a momentary current causes the voltage to rise. Volts times electron-flow equals energy-flow ( V x I = P). Therefore during a momentary current through a capacitor, there is a joules-per-second transfer of energy from the power supply into the capacitor.
Therefore, during the "charging" process, ENERGY is placed in the capacitor. Capacitors store energy, not charge. When we "charge" a capacitor, we give it a charge of energy. Because we use the word "charge" to refer both to electric charges and ALSO to quantities of energy, capacitor explanations are impossible to understand. "Charging" a capacitor means injecting electrical energy into the device.

Similar trouble is caused when we say that we "charge" a battery. We charge a battery with some energy in the form of stored chemical fuel, but we pump electric charge THROUGH the battery and none of it builds up inside.


It's all terribly confusing. What are students to think if we tell them that "charging a battery" does not store any charge, yet charge must flow through the battery if we want to charge it! Ugh. The word "charge" has far too many meanings. In science this is always a Very Bad Thing.


Another, less misleading situation is similar: think of the word "charge" as applied to gunpowder. A charge is placed in an old cannon, followed by a cannonball. It would be silly to assume that, because we've "charged" the cannon, the cannon now has an electrical charge. But whenever we state that we've "charged" a capacitor, we DO assume that an electrical charge has been stored inside. This is just as silly as mistaking gunpowder for electrostatic charges. Charging a capacitor is like charging a cannon; in both situations we are inserting energy, not electrical charge.

amasci.com...

[edit on 14-5-2007 by junglelord]

I've never really considered the difference between the words 'charge' and 'charge.'

But indeed, when the same word means different things in science mistakes are bound to happen. I like the analogy with the cannon at the bottom. Made it all clear.

I absolutely did not click the post button twice. WTF?

[edit on 14/5/2007 by watch_the_rocks]

Yeah, this is definitely an issue with miss-use of English words. This is how slang comes into play, even in the technical world.

The problem though, is that it takes too long to say "Induce an electrical potential into X device" when we could save time and say "Charge X device" and people still know what you meant.

When you 'charge' a capacitor, you are merely creating a buildup of electrical potential as opposed to the other plate which has a lack of energy. Hence you're just creating an electrical potential. A charge would suggest that the energy is still flowing even when it's disconnected.

But hey, such is the evolution of a language to make it easier to use. If we're intelligent to know what the word means in different circumstances, we can allow the slang.

Cells and Capacitors work differently, but they are both charged, with the word meaning the same thing.

A cell will provide power through a chemical process, whereas a capacitor holds on one plate a buildup of electrons.

The cell can maintain a constant output of energy until it's chemical components are no longer able to, whereas a capacitor discharges it's electrons once a given value of PD is reached.

It is true to say a capacitor builds up a charge, as electrons do mass on one plate within the capacitor, until they have accrued enough PD to make the jump to the other plate.

You also charge a battery by pumping free electrons into it, which enables the ions within to retain a given amount of energy. It is different to capacitors as it releases the energy in a steady stream, whereas on a capacitor, once the intitial charge has left, there is no means to maintain a current without charging it once more to the given PD value so the electrons can jump again.

The word means the same thing, ie; your pumping electrons in. What the electrons do once there is different between a cell and capacitor, but that doesn't change the fact both are charged.